A Simple Scala Call-by-name Example

Here's a simple Scala call-by-name example. I ' ll show the normal approach to writing a method and passing in a parameter, and then show a call-by-name (pass by name) Example. 1) A "Normal" Scala method

Here I show how to pass a parameter to a method "normally", i.e., call by value:

Object Test extends App {

  def time (): Long = {
      println ("In Time ()")
      System.nanotime
  }

  def exec (T:lon g): Long = {
      println ("entered exec, calling t ...")
      println ("t =" + t)
      println ("Calling T again ...")
      t
  }

  println (EXEC (Time ()))

}

The output looks like this:

In time ()
entered exec, calling T
... t = 1363909521286596000
calling T again
... 1363909521286596000

As expected, the values for T is the same. This is because the value of T was determined when this line is invoked:

println (EXEC (Time ()))

2) A call by name example

Next, make the method parameter a call by name parameter:

Object Test extends App {

  def time () = {
      println ("Entered time () ...")
      system.nanotime
  }

  //uses a by- Name parameter here
  def exec (t: = = Long) = {
      println ("entered exec, calling t ...")
      println ("t =" + t) 
  println ("Calling T again ...")
      t
  }

  println (EXEC (Time ()))

}

This time, the output is different:

Entered exec, calling T
... Entered time ()
... t = 1363909593759120000
calling T again
... Entered time ()
... 1363909593759480000

The both T invocations yield different results. As stated in the Scala language specification, this is because:

"This indicates the argument are not evaluated at the point of function application, but instead are evaluated at each Use within the function. That's, the argument is evaluated Usingcall-by-name. "

That's a simple call by the name example in Scala.